Antimony plating bath



Un t d States, Pstsffi F? ANTIMONY PLATING BATH Clarence F. Smart, Birmingham, Mich., assignor to General Motors Corporation, Detroit, Micln, a corporation of Delaware No Drawing. Application October 23, 1953 Serial No. 388,059

3 Claims. (Cl. 204-45) This invention relates to improvements in the electro deposition of antimony and more particularly to an improved ele-ctrolyte and its use in the electrodeposition of antimony.

It has heretofore been difficult to obtain satisfactory electrodeposits of antimony. Not only have prior plating procedures produced antimony coatings which are poorly adherent and difiicult to buff or polish, but frequently the electrodeposited antimony has been unsatisfactory because of pitting and other surface defects. Moreover, prior electrolytic solutions employed in antimony plating have generally exhibited poor stability which has heretofore limited when the prior solutions intermittently.

I have found that the aforementioned difiiculties are minimized and in many instances eliminated entirely when antimony is deposited from an aqueous solution of dis solved antimony and an alkaline citrate such as sodium citrate, lithium citrate, potassium citrate or ammonium citrate. In most instances it is desirable that the bath also contain small amounts of citric acid and/or sodium benzoate or sodium propionate.

The following examples in which the quantities are expressed in ounces per gallon of water are illustrative of have only been used plating baths embodying the present invention.

Example I Sodium citrate 20.

Citric acid 6.

Antimony potassium tartrate (3.59 metallic antimony).

Example Il Potassium citrate 25.

Citric acid 6.

Antimony potassium tartrate 12 (4.3 metallic antimony).

Example 111 Ammonium citrate (dibasic); 8.

Antimony potassium tartrate 1.5 (.54 metallic antimony).

Sodium benzoate .5.

Example IV Lithium citrate 14.

Citric acid 2.

Antimony potassium tar-trate 10 (3.59 metallic antimony). Sodium benzoate .5.

While the above examples illustrate specific plating baths embodying the invention, it is to be understood, of course, that the invention is not to be limited by these examples because I have found that excellent plating results are obtained when an alkaline citrate is employed in an amount within the range of about 5 to 26 ounces per gallon of water. In certain applications the quantity of alkaline citrate can be increased, the upper limit being determined by the limit of solubility of the alkaline citrate in the bath. Similarly, the minimum amount of alkaline citrate to be used in any particular application is that amount necessary to maintain adequate anode corrosion during plating. In a like manner, satisfactory results are obtained when the citric acid is present in an amount within the range of approximately 2 to 7 ounces per gal lon of Water and dissolved antimony is present in an amount within the range of about .5 to 5 ounces per gallon of metallic antimony, i. e. approximately 2 to 12 ounces per gallon of antimony potassium tartrate. Although the sodium benzoate may be omitted in certain applications, superior results are obtained when it is present in an amount not in excess of about 3 ounces per gallon of water, /2 ounce per gallon of water being preferred at present.

If desired, the antimony potassium tartrate can be partially or completely replaced with other soluble antimony salts such as the chloride, acetate or sulfate. Similarly, the sodium benzoate may be replaced with sodium propionate.

their useful life, especially Plating baths in accordance with this invention generally may be operated over a wide range of temperatures from room temperature up to a temperature determined primarily by the economic considerations of minimizing heating costs and evaporation losses. However, I have found that it is generally desirable to operate the bath at an elevated temperature within the range from approximately F. to 150 F., F. being preferred at present. The current density may be varied in different applications although I have found that a current density within the range of about 25 to 30 amperes per square foot produces excellent results in most instances.

I have found that baths embodying the present invention operate most satisfactorily as an acid medium and produce superior results at a pH within the range of approximately 4 to 6, 5 being preferred. In most applications I prefer to employ antimony anodes. It will be understood of course that in certain instances it may be desirable to use inert anodes and to maintain the desired concentration of antimony in the plating bath by separate additions of soluble antimony salts.

Plating baths of the present invention may be used to deposit antimony on any conventional cathode material such as lead, indium, alloys of lead and indium, copper, iron and steel, zinc, tin, cadmium, lead-antimony alloys or antimony-lead alloys.

At times, as in the electrodeposition of antimony over lead when: it is desired to improve the adhesion of antimony to the lead and to minimize resistance to blistering in subsequent cleaning and plating operations, it is desirable to initially deposit a thin flash coat of antimony over the cathode. For that purpose I have found that the following bath composition, in which the quantities are expressed in terms of ounces per gallon of water, provides excellent results:

Sodium citrate 10-12 Citric acid 3-4 Antimony potassium tartrate 2-2%a Patented Dec. 24,1957

3 metallic article is first coated with lead, indium, or a lead-indium alloy to protect the article from corrosion in subsequent plating baths. This protective coating preferably has a smooth surface. mony is then electrodeposited over the protective coating from a plating bath comprising an aqueous solution of to 12 ounces per gallon of sodium citrate, 3 to 4 ounces per gallon of citric acid, 2 to 2.5 ounces per gallon of antimony potassium tartrate and .5 ounce per gallon of sodium benzoate. A subsequent thicker coating of antimony is then electrodeposited using a bath comprising an aqueous solution of 18 to ounces per gallon of sodium citrate, 6 to 7 ounces per gallon of citric acid, 10 to 12 ounces per gallon of antimony potassium tartrate and .5 ounce per gallon of sodium benzoatc.

The resultant structure is then buffed or polished, it' necessary, and may be used without further treatment. On the other hand, the article may be subjected to a further electrodeposition process for the application of other metals such as nickel and/ or chromium for which an electrodeposited coating of antimony formed in accordance with the present invention is an excellent undercoat or base material.

It is to be understood that although the invention has been described With specific reference to particular embodiments thereof, it is not to be so limited since changes and alterations therein may be made which are Within the full intended scope of this invention as defined by the appended claims.

What is claimed is:

1. A process for electrodepositing antimony, said process comprising the steps of establishing a flow of electricity from an antimony anode to a cathode through an electrolyte at a current density of approximately to A flash coating of anti asraeaa 30 amperes per square foot while said electrolyte is at a temperature within the range of approximately F. to F., and is maintained at a pH within the range of about 4 to 6, said electrolyte consisting essentially of an aqueous solution of approximately 18 to 20 ounces per gallon of sodium citrate, 6 to 7 ounces per gallon of citric acid, 10 to 12 ounces per gallon of antimony potassium tartrate and .5 ounce per gallon of sodium benzoate.

2. An electrolytic bath consisting essentially of an aqueous solution of antimony potassium tartrate, an alkaline citrate, citric acid, and a substance selected from the group consisting of sodium propionate and sodium benzoate, the concentrations of the several ingredients per gallon of water being substantially as follows: antimony potassium tartrate 2 to 12 ounces, an alkaline citrate 5 to 26 ounces, citric acid 2 t0 7 ounces, and a substance selected from the group consisting of sodium benzoate and sodium propionate about 0.5 to 3 ounces.

3. A process of electrodepositing antimony which comprises passing electric current from an anode to a cathode through the bath of claim 2.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A PROCESS FOR ELECTRODEPOSITING ANTIMONY, SAID PROCESS COMPRISING THE STEPS OF ESTABLISHING A FLOW OF ELECTRICITY FRON ANTIMONY ANODE TO A CATHODE THROUGH AN ELECTROLYTE AT A CURRENT DENSITY OF APPROXIMATELY 25 TO 30 AMPERES PER SQUARE FOOT WHILE SAID ELECTROLYTE IS AT A TEMPERATURE WITHIN THE RANGE OF APPROXIMATELY 125* F. TO 140*F., AND IS MAINTAINED AT A PH WITHIN THE RANGE OF ABOUT 4 TO 6, AND ELECTROLYTE CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION OF APPROXIMATELY 18 TO 20 OUNCES PER GALLON OF SODIUM CITRATE, 6 TO 7 OUNCES PER GALLONON OF CITRIC ACID, 10 TO 12 OUNCES PER GALLON OF ANTIOMONY POTASSIUM TARTRATE AND .5 OUNCES PER GALLON OF SODIUM BENZOATE. 